At present, optical disks such as CDs and DVDs are available and optical disks such as BDs and HD-DVDs for achieving higher density recording by means of a blue laser are available. Optical disks having two-layer recording surfaces have been put into practical use as DVDs and BDs. Further, in recent years, recordable and reproducible optical disks with multilayer recording surfaces of four layers and eight layers are proposed to further increase capacities (for example, patent document 1).
[Patent document 1]: (Japanese Patent Laid-Open No. 2004-213720)